Organic light-emitting diodes, based on self-luminescence, exhibit the advantages of having a wide viewing angle, excellent contrast, fast response time, high brightness, excellent driving voltage and response rate characteristics, and of allowing for a polychromic display.
A typical organic light-emitting diode includes a positive electrode (anode) and a negative electrode (cathode), facing each other, with an organic emission layer disposed therebetween.
As to the general structure of the organic light-emitting diode, a hole transport layer, a light-emitting layer, an electron transport layer, and a cathode are formed in that order on an anode. Here, all of the hole transport layer, the light-emitting layer, and the electron transport layer are organic films comprising organic compounds.
An organic light-emitting diode having such a structure operates as follows: when a voltage is applied between the anode and the cathode, the anode injects holes which are then transferred to the light-emitting layer via the hole transport layer while electrons injected from the cathode move to the light-emitting layer via the electron transport layer. In the luminescent zone, the carriers such as holes and electrons recombine to produce an exciton. When the exciton returns to the ground state from the excited state, the molecule of the light-emitting layer emits light.
Materials used as the organic layers in organic light-emitting diodes may be divided according to functions into luminescent materials and charge carrier materials, for example, a hole injection material, a hole transport material, an electron injection material, and an electron transport material. The light-emitting mechanism forms the basis of classification of luminescent materials as fluorescent and phosphorescent materials, which use excitons in singlet and triplet states, respectively.
Meanwhile, when a single material is employed as the luminescent material, intermolecular actions cause the maximum luminescence wavelength to shift toward a longer wavelength, resulting in a reduction in color purity and light emission efficiency due to light attenuation. In this regard, a host-dopant system may be used as a luminescent material so as to increase the color purity and the light emission efficiency through energy transfer. This is based on the principle whereby, when a dopant which is smaller in energy band gap than a host forming a light-emitting layer is added in a small amount to the light-emitting layer, excitons are generated from the light-emitting layer and transported to the dopant, emitting light at high efficiency. Here, light with desired wavelengths can be obtained depending on the kind of the dopant because the wavelength of the host moves to the wavelength range of the dopant.
With regard to the efficiency of organic light-emitting diodes, statistically, the probability of forming a singlet state and a triplet state occurs at a ratio of 25%:75%. It would thus be expected that in fluorescent EL devices, only the formation of singlet excitons results in the emission of useful radiation, placing a theoretical limit of 25% on the internal quantum efficiency of such fluorescent EL devices %.
With the aim of solving the disadvantages, Korean Patent No. 10-2012-0092555 A (Aug. 21, 2012) proposes the effective occurrence of a triplet-triplet fusion (TTF) phenomenon accounting for the generation of singlet excitons through the collision and fusion of two triplet excitons. For this, this document discloses an electroluminescence device in which a blocking layer is interposed between a light-emitting layer and an electron injection layer, with an affinity difference between the electron injection layer and the blocking layer. In this regard, the blocking layer is set to have a triplet energy larger than that of the host of the light-emitting layer so as to confine triplet excitons within the light-emitting layer, whereby the effective occurrence of the TTF phenomenon is induced.
As described above, the disclosure set forth in the reference document is designed to cause the effective occurrence of a TTF phenomenon in order to provide high emission efficiency for an organic electroluminescence device. To this end, the blocking layer should include a material that is higher in triplet energy than the host to prevent the annihilation of the triplet excitations generated in the host, and an aromatic heterocyclic compound of a specific fused ring should be employed in the blocking layer.
Another technique for improving luminance efficiency can be found in Korean Patent No. 10-2006-0022676 A (Mar. 10, 2006), which describes an organic electroluminescence device having a blocking layer, disposed between a light-emitting layer and an electron transport layer, for controlling electron density.
In spite of various efforts made to fabricate organic light-emitting diodes having effective luminescence characteristics, however, there is still a continued need to develop organic light-emitting diodes having a higher effective luminance efficiency.